The invention relates to a method of reinforcing cast thermoplastic structures by utilizing continuous fiber reinforcing elements and the articles made thereby.
The use of discontinuous or chopped fibers in reinforcing thermoplastic resins for engineering applications has long been established. In general, the short fibers cost the least, and fabrication costs are lowest. However, the properties of the resulting composite are relatively low. The primary use of chopped fiber reinforced thermoplastic resins has been in the manufacture of injection molded parts which are typically relatively small and must be relatively thin in cross-section such as relatively small gears, bearings, bushings, thrust washers and the like small articles. Since thermoplastic resins typically are poor heat conductors, injection molds are often chilled to speed the solidification process. With relatively thick cross-section parts, cooling can result in rapid surface hardening while the center of the part is yet molten. Defects such as voids, sinks and distortion become inevitable under such conditions. Thus, injection molding with reinforcing fibers has been limited to articles with relatively thin cross-sections.
Extrusion of reinforced thermoplastic shapes is also possible although distribution of the reinforcing fibers throughout the thermoplastic resin is not complete and there is a practical limit to the size of extruded thermoplastic shapes.
The process of casting thermoplastic resins according to the present invention extends the size range of stock shapes or custom shapes of articles well beyond that possible with injection molding or extrusion and enables the manufacture of relatively thick cross-sectional shapes. However, previous attempts to reinforce cast thermoplastic materials with milled or chopped fibers has failed due to settling of the reinforcement fibers before the thermoplastic solidifies. Further, attempts to produce thermally or electrically cnductive cast thermoplastics by the addition of conductive reinforcements has also failed due to encapsulation of the chopped, discontinuous reinforcement material.
Continuous strands of reinforcement have been utilized for reinforcing thermosetting resins, however, such has usually been in fabric forms wherein strands are oriented in the weft and warp directions limiting flexibility in arranging of the lines of fiber reinforcement directionally in the structure. The elements in the weave pattern undulate which lessens the strength as compared to a straight element. U.S. Pat. No. 3,870,580 discloses a structure reinforced with woven reinforcing fibers.
The most common form of construction for reinforced thermosetting resins is the lay-up process where the fiber reinforcing elements are not anchored but set and held in place by the addition of the resin binder during lay-up. Reinforcement of thermosetting resinous structures has been limited generally to that of fabrics owing to the necessity of setting the reinforcing fabric in place by chemical binding.